Recently, a surface emitting laser element (surface emitting semiconductor laser element) which emits laser beams in a direction perpendicular to a surface of a substrate of the surface emitting laser element has been greatly researched. In the surface emitting laser element, an oscillation threshold current is lower than that of an edge emitting laser element, and a high-quality circle-shaped laser beam can be obtained. In addition, since the surface emitting laser element can emit a laser beam in a direction perpendicular to a substrate of the surface emitting laser element, the surface emitting laser elements can be two-dimensionally arrayed with high density. Therefore, applications to, for example, a light source for interconnecting parallel light and a high-speed electrophotographic system having high definition have been studied.
In order to increase current injection efficiency, the surface emitting laser element has a confinement structure. As the confinement structure, a confinement structure in which Al (aluminum) is selectively oxidized is frequently used. Hereinafter, in some cases, this confinement structure is refereed to as an oxide confinement structure (see Non-Patent Documents 1 and 2).
In addition, in Non-Patent Document 3, a printer using a VCSEL (vertical cavity surface emitting laser) array (surface emitting laser array) of a 780 nm band is described.
In addition, Patent Document 1 discloses a surface emitting laser element. In the surface emitting laser element, a difference (detuning amount) between an oscillation wavelength determined by the length of a resonator and a gain peak wavelength determined by the composition of an active layer is a predetermined amount at a predetermined temperature, and the oscillation wavelength becomes equal to the gain peak wavelength at a temperature range higher than the predetermined temperature.
In addition, Patent Document 2 discloses a multi-spot image forming apparatus having a multi-spot light source.
[Patent Document 1] Japanese Unexamined Patent Publication No. 2004-319643
[Patent Document 2] Japanese Unexamined Patent Publication No. H11-48520
[Non-Patent Document 1] K. D. Choquette, K. L. Lear, R. P. Schneider, Jr., and K. M. Geib, “Cavity Characteristics of Selectively Oxidized Vertical-Cavity Lasers”, Applied Physics Letters, vol. 66, No. 25, pp. 3413-3415, 1995
[Non-Patent Document 2] K. D. Choquette, R. P. Schneider, Jr., K. L. Lear, and K. M. Geib, “Low Threshold Voltage Vertical-Cavity Lasers Fabricated by Selective Oxidation”, Electronic Letters, No. 24, vol. 30, pp. 2043-2044, 1994
[Non-Patent Document 3]H. Nakayama, T. Nakamura, M. Funada, Y. Ohashi, and M. Kato, “780 nm VCSELs for Home Networks and Printers” Electronic Components and Technology Conference Proceedings, 54th, vol. 2, June 2004, pp. 1371-1375
In electrophotography, a rise movement of a response pulse shape of light output from a light source greatly influences image quality when a driving current is supplied to the light source. The response pulse shape is a change of the light output with the passage of time, and in some cases is referred to as a light pulse shape. For example, not only at a rise time of the light pulse shape but after the light output reaches a constant amount at the initial rise time, even if the light amount is slightly changed, there is a risk that the image quality may be lowered.
It is a contour part of an image that is formed at a rise time and a fall time of the light pulse shape, and in particular at the rise time and for a few moments after that is regarded as approximately risen; when the light amount is changed, the contour part of the image becomes unclear and image quality becomes visually indistinct.
For example, when a time required to scan one line of approximately 300 mm width of A4 (297 mm×210 mm) size paper is 300 μs, a width of approximately 1 mm is scanned in 1 μs. It is said that a width of 1 to 2 mm is visually perceived highest by a person when image density is changed. Therefore, when the image density is changed in the width of approximately 1 mm, the image density change is sufficiently detected by the person and the image gives the person an impression that the contour of the image is indistinct.
FIG. 39 is a graph showing a light pulse shape of a conventional surface emitting laser element. In FIG. 39, a surface emitting laser element having an oxide confinement structure is driven by pulse conditions in which the pulse width is 500 μs and the duty ratio is 50% (pulse period is 1 ms). As shown in FIG. 39, when viewed from a relatively long time scale, the light output shows a peak right after the rise, and is lowered and becomes stable. The change of the light output is caused by self-heating of the surface emitting laser element and is called “droop characteristics”.
FIG. 40 is an enlarged view of the rise part of the light output shown in FIG. 39. The inventors of the present invention have studied the light output in detail. As shown in FIG. 40, when viewed from a short time scale, the inventors have found generation of a change of the light output different from the droop characteristics.
In FIG. 40, the light output has not become a risen state even when 10 ns has passed, becomes an approximately risen state after passing approximately 200 ns, and gradually increases until the time reaches approximately 1 μs. This phenomenon (characteristics) has been newly found by the inventors of the present invention. In this description of the present invention, the characteristics are called “negative droop characteristics”. The negative droop characteristics are not generated in the conventional edge emitting laser element.
The inventors of the present invention have focused on the negative droop characteristics and have studied the reasons for the negative droop characteristics in detail. It has been found that the negative droop characteristics closely relate to the intensity of the light confinement in the lateral mode of the oxide confinement structure. In addition, it has been found that a surface emitting laser element in which the negative droop characteristics are prevented, that is, a surface emitting laser element having good negative droop characteristics may have a short service life.